This invention relates to an optical sensor for detecting Beryllium ions and/or measuring concentration of the same, and a method for enabling the optical sensor in performing said functions.

BACKGROUND OF THE INVENTION

Beryllium (“Be”) being an alkali metal is indispensable in a variety of high technology industries, such as automobile manufacturing, aerospace, nuclear energy, electronics and communications. Contrary to the utility that Be offers, it is categorised as a Group A carcinogen and known to be one of most toxic elements extremely hazardous to human health. As the above-mentioned industries flourish, more and more workers in the industries or civilians residing in the vicinity are exposed to the danger of Be. Such exposure, which could occur in the process of manufacturing, production or disposal of waste, leads to chronic Beryllium diseases (CBD), acute clinical pneumonitis, and lung or bone cancers. As such it is necessary to detect Be and remove it from the environment in which it is disposed of.

Various techniques have been introduced for the detection of Be. These techniques include Gas Spectrometry, Atomic Absorption Spectrometry (“AAS”), Inductively Coupled Plasma Mass Spectrometry (“ICP-MS”), and Inductively Coupled Plasma Atomic Emission Spectrometry (“ICP-AES”). Nonetheless, the above-mentioned techniques suffer from a number of drawbacks including a requirement for very expensive instruments and taking a relatively long time to be completed. Said drawbacks further hinder the techniques from being employed for on-site, in-situ detecting and measurement of Be.

In a patent application no. CN104003370A, a method for preparing a fluorescent carbon quantum dot probe has been disclosed for detecting Beryllium in water. Beside the fluorescent carbon quantum dot, macrocyclic compounds have been known to be able to bind metal cations in a contaminated medium. Nonetheless, there is a difficulty in coating a specific macrocyclic compound on a test structure, more especially on a compact test structure that can be employed for detecting Be and measuring concentration of the same in an on-site, in- situ manner.

SUMMARY OF THE INVENTION

The above-mentioned drawbacks and difficulty are overcome by the invention elaborated in the paragraphs that follow.

One aspect of the invention provides an optical sensor for detecting Beryllium ions and/or measuring concentration of the same in a fluid medium, comprising a resonant structure, a surface of at least part of which is covered by a layer of a macrocyclic compound, characterised in that said macrocyclic compound is diamino-benzo-9-crown-3, the material index of which changes in accordance with the amount of Beryllium ions bound thereby.

Advantageously diamino-benzo-9-crown-3 is an ionophore capable of chemically binding Beryllium ions in the medium, such that the consequential change in the material index can be detected and/or measured.

Advantageously the resonant wavelength of said resonant structure shifts in accordance to the change in the material index.

In one embodiment, said resonant structure comprises a test waveguide and a reference waveguide, said test waveguide being exposable to the medium, and said reference waveguide being sealed from any contact with the medium. Typically the resonant structure is formed in silicon or silicon dioxide.

In one embodiment, a light source is configured to emit a light of at least one wavelength which is directed through said test and reference waveguides, the shift in resonant wavelength being determined by comparing the patterns generated from the respective test and reference waveguides, said shift corresponding to the concentration of Beryllium ions in the medium. Advantageously the comparison allows shifts of <500pm to be measured accurately, enabling a sensitivity down to lOppb Beryllium ions.

In yet another embodiment, the optical sensor also comprises a spectral interrogator, connected to the outputs of said test and reference waveguides, capable of providing a reading in wavelength or phase of the light. Typically, each of said test and reference waveguides is of 15-1000nm in width and 200pm to 1cm in length.

Typically, the light source comprises at least one laser source, and is capable of emitting light at a wavelength ranging from 1500 to 1600 nm. Typically the laser source is able to sweep through and emit light of the aforementioned wavelength range in less than a second.

Typically, during the test said exposable resonant structure is exposed to the medium for a time interval of 30 seconds to 2 minutes.

In yet another embodiment, the optical sensor also comprises a pair of multi-mode interferometers, one of which is connected to an input end of said resonant structure for diverging the light, and the other one is connected to the output end of said resonant structure for converging the light.

In yet another embodiment, the optical sensor also comprises: a silicon slab interposer and a spot size converter connected to said multi-mode interferometer at the input end; and a waveguide, an input of which is connected to the light source, and output of which is connected to said silicon slab interposer.

In yet another embodiment, said spectral interrogator comprises: a pair of multi- mode-interferometer-based reflectors and two pairs of micro-ring resonators.

Typically, the resonant wavelength of said resonant structure shifts over time as said macrocyclic compound is exposed to a medium with a composition of different metal ions.

Typically, said shift of resonant wavelength is a superimposition of the curves of resonant wavelength versus time for each metal ion in the composition, wherein said superimposition is usable in training an artificial neural network to determine presence of Beryllium ions and/or concentration of the same in a medium.

Advantageously said macrocyclic compound has unique kinetics in absorbing Beryllium ions, hence causes the resonant structure to provide a distinct curve of resonant wavelength versus time as said macrocyclic compound is exposed to a medium with a concentration of Beryllium ions, wherein said distinct curve is useable for identifying Beryllium ions and/or measuring concentration of the same, in the medium. Another aspect of the invention provides a method for covering a surface of a resonant structure of an optical sensor with a layer of a macrocyclic compound such that the sensor is capable of detecting Beryllium ions and/or measuring concentration of the same in a fluid medium, comprising steps of: performing plasma treatment on the surface using oxygen gas; and treating the surface using a 2% (v/v) 3-Aminopropyltriethoxysilane (APTES) solution diluted in pure ethanol, at room temperature for a first predetermined time interval characterised by treating the surface using 50 ml of 100 mM diamino-benzo-9-crown-3 (DAB9C3) solution diluted in a solvent; and leaving the surface undisturbed for the solvent in the DAB9C3 solution to evaporate, such that the layer of macrocyclic compound is formed on the surface.

Typically, the macrocyclic compound is diamino-benzo-9-crown-3.

Diamino-benzo-9-crown-3 is an ionophore capable of chemically binding Beryllium ions in the medium, such that material index of the diamino-benzo-9- crown-3 is changed in accordance with amount of Beryllium ions bound thereby.

In one embodiment, the method also comprises steps of: drying the surface at 80°C for the first predetermined time interval; and treating the surface using a 0.1% (v/v) glutaraldehyde (GA) solution diluted in deionized water, at room temperature for a second predetermined time interval.

Typically, the first predetermined time interval is 1 hour, and the second predetermined time interval is 20 minutes.

Typically, the solvent is methanol.

Typically, the optical sensor comprises a Mach-Zehnder interferometer.

Yet another aspect of the invention provides an optical sensor fabricated by the above-mentioned method.

Advantageously resonant wavelength of said resonant structure shifts in accordance to the change in the material index.

Typically, the resonant wavelength of said resonant structure shifts over time as said macrocyclic compound is exposed to a medium with a composition of different metal ions, said shift of resonant wavelength being a superimposition of the curves of resonant wavelength versus time for each metal ion in the composition, wherein said superimposition is usable in training an artificial neural network to determine presence of Beryllium ions and/or concentration of the same in a medium.

Advantageously said macrocyclic compound has unique kinetics in absorbing Beryllium ions, hence causes the resonant structure to provide a distinct curve of resonant wavelength versus time as said macrocyclic compound is exposed to a medium with a concentration of Beryllium ions, wherein said distinct curve is useable for identifying Beryllium ions and/or measuring concentration of the same, in the medium.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawing, in which:

Fig. 1 shows the mechanism, according to the invention, by which Beryllium cations are captured by a layer of macrocyclic compound coated on the surface of a resonant structure;

Fig. 2 shows that the resonant wavelength of said resonant structure shifts over time as said macrocyclic compound is exposed to a medium with a composition of different metal ions;

Fig. 3 shows a wavelength of the optical wave, higher than that when the resonant structure is not coated with the layer of macrocyclic compound, is required to cause the resonant structure to resonate;

Fig. 4 is a plan view of a dielectric planar light-wave circuit (PLC) with waveguide sections of different widths;

Fig. 5 is top view of a sensor chip, input of which is connected to a laser source and output of which is connected to a spectral interrogator;

Fig. 6 is top view of a Mach-Zehnder interferometer system constructed on a silicon on insulator platform; and

Fig. 7 is a graph of transmission versus resonant wavelength showing shifts of the resonant wavelength as the concentration of Be ²⁺ in a medium is increased. Certain macrocyclic compound serves as an effective binding site for Be cations (“Be ²⁺ ”), hence can be used for the separation or removal of Be ²⁺ from a contaminated medium. The ability to bind Be ²⁺ attributes to a good match between the cavity of the macrocyclic compound and the ionic radius of the Be ²⁺ . Factors which effect recognition of the Be ²⁺ , and stability and selectivity of the recognition, include the cavity dimension, shape, substituent effect, conformational flexibility, type of donor atom, and the solvent of the macrocyclic compound. A macrocyclic compound capable of binding Be ²⁺ provides a cavity that a Be ²⁺ closely fits into, and a set of oxygen atoms capable of arranging around the Be ²⁺ in a nearly planar manner, hence yields a promising result in selectively separating Be ²⁺ from mediums such as complex water and nuclear waste solution.

Referring to Fig. 1, one form of the optical sensor, which can be a Mach-Zehnder interferometer, comprises a resonant structure 1, e.g. a waveguide, the surface of which is coated by a layer of macrocyclic compound 2, i.e. diamino-benzo-9- crown-3 (“DAB9C3”). DAB9C3 is an ionophore capable of chemically binding Be ²⁺ in a fluid medium, e.g. liquid or gas, and forming a complex 3 consisting of DAB9C3 and Be ²⁺ .

The method for coating surface of the resonant structure 1 with the layer of macrocyclic compound 2 comprises the following steps. First, for the purpose of hydroxylating the surface, a plasma treatment is performed on the surface using oxygen gas. Then the surface is treated using a 2% (v/v) 3- Aminopropyltriethoxy silane (“APTES”) solution diluted in pure ethanol, at room temperature for 1 hour. Subsequently, the surface is completely dried at 80°C for 1 hour, after which the surface is treated using a 0.1% (v/v) glutaraldehyde (“GA”) solution diluted in deionized water, at a room temperature for 20 minutes. The surface is then treated using 50 ml of 100 mM diamino-benzo-9-crown-3 (“DAB9C3”) solution diluted in a solvent, i.e. methanol. Last, the surface is left undisturbed for the solvent in the DAB9C3 to evaporate, such that the layer of macrocyclic compound 2, i.e. DAB9C3, is formed or immobilized on the surface.

The coated resonant structure 1 is functionalized by DAB9C3 which acts as a ligand for Be ²⁺ , hence can detect divalent cations of Be. When the coated resonant structure 1 is exposed to a medium which carries Be ²⁺ , the material index of the DAB9C3 is changed in accordance with the amount of Be ²⁺ bound by the DAB9C3.

A wider range of detection for Be ²⁺ can be achieved when the concentration of the DAB9C3 immobilized on the surface is increased. As such the concentration of the DAB9C3 can be adjusted to optimize said detection range in accordance with a test requirement.

As the layer of DAB9C3 is heterogeneously integrated with the resonant structure 1, a change in the material index modifies the resonant condition of the surface of the resonant structure. As shown in Fig. 2, the resonant wavelength of the resonant structure shifts over time as said macrocyclic compound is exposed to a medium with a composition of different metal ions, said shift of resonant wavelength being a superimposition of the curves of resonant wavelength versus time for each metal ion in the composition. The superimposition is usable in training an artificial neural network to determine presence of Beryllium ions and/or concentration of the same in a medium. Said macrocyclic compound has unique kinetics in absorbing Beryllium ions, hence causes the resonant structure to provide a distinct curve of resonant wavelength versus time as said macrocyclic compound is exposed to a medium with a concentration of Beryllium ions. Said distinct curve is useable for identifying Beryllium ions and/or measuring concentration of the same, in the medium.

The higher the concentration of the Beryllium ions, the faster the resonant structure achieves the maximum resonant wavelength corresponding to said concentration.

A material index is a grouping of material properties which affect the characteristic of the layer. The properties include relative dielectric constant, thermal resistivity (°C.cm/W), and loss tangent. For a silicon photonics sensor, the material index is usually the refractive index.

Resonance can be defined as a vibration of large amplitude in a mechanical, electrical or optical system caused by a relatively small periodic stimulus of the same or nearly the same period as the natural vibration of the system.

For the optical sensor, resonance occurs at its resonant structure, and the periodic stimulus is an optical wave of a specific wavelength. The specific wavelength is known as the resonant wavelength, namely a wavelength of the optical wave by which the resonant structure is caused to resonate.

The addition of a macrocyclic compound layer to the surface of the resonant structure changes the condition under which the resonant structure resonates. As shown in Fig. 3, a wavelength of the optical wave, higher than that when the resonant structure is not coated with the layer of macrocyclic compound, is required to cause the resonant structure to resonate. As the concentration of the Be ²⁺ increases, the material index of the layer of macrocyclic compound changes, and the resonant wavelength of the resonant structure increases accordingly.

The resonant structure is an essential part of the optical sensor where the macrocyclic compound is capable of influencing the wavelength of the light passing through the structure. As the light passes through the structure, an electric field travels beyond the wall of the structure as evanescent field. The evanescent field is susceptible to the surrounding, hence is easily influenced by the layer of macrocyclic compound residing on the outer surface of the structure. As the material index of the macrocyclic compound changes due to the absorption of the Be ²⁺ , the evanescent field is affected by the changes, and the resonant wavelength of the light passing through the structure increases accordingly.

Referring to Fig. 4, the optical sensor can be a dielectric planar light-wave circuit ("PLC") with waveguide sections of different widths, typically arranged in pairs. Each end of the PLC has a fibre array assembly, to which a fibre adaptor is connected. The fibre adaptor adapts an optical fibre to the PLC and vice versa. Said circuit can be mass-produced by a semiconductor wafer process. At least one of the waveguide sections is coated with the layer of macrocyclic compound. This coated waveguide section functions as a resonant structure of the PLC.

Referring to Fig. 5, multiple PLCs can be deployed on a sensor chip 4 of a scale of one or more mm ² . For each PLC, one of the waveguide sections is exposed 5, while the other one is sealed 6 from any contact with a medium (or is not coated with the macrocyclic compound). During a test, the exposed section 5 provides a wavelength corresponding to the concentration of Be ²⁺ in the medium, whereas the sealed section 6 provides a reference wavelength corresponding to a condition in which Be ²⁺ is of non-existence. The input of the PLC is connected to a plurality of laser sources 7, and the output of the PLC is connected to spectral interrogator 8 which functions as the detector for Be ²⁺ .

Referring to Fig. 6, the resonant structure can also be deployed in a Mach-Zehnder interferometer (“MZI”) system which is constructed on a silicon-on-insulator (“SOI”) platform. Input of the MZI system is a III-V waveguide 9 which is connected to a silicon slab interposer and spot size converter (“SSC”) 10. The interposer has a silicon dioxide SiO2 cladding, and is of 6pm in width and 0.07pm in thickness. Output of the interposer and SSC is coupled to a 1x2 multi-mode interferometer (“MMI”) 11, output of which is diverged to an unbalanced MZI 12 and waveguide sensing section 13, latter of which is the resonant structure having a surface coated with a layer of DAB9C3. The waveguide sensing section 13 has a silicon dioxide SiO2 cladding, and is of 15-1000nm in width, 220nm in thickness and 200pm to 1cm in length. The other ends of the unbalanced MZI 12 and waveguide sensing section 13 converge to a 2x1 MMI 14 which couples them to an interrogator 15 fully integrated in the system. The MMIs 11, 14 deployed at said two ends are vertical grating couplers in which transverse mode (“TM”) is preferred. The interrogator 15 comprises a pair of MMI-based reflectors 16, 17 and 2 pairs of micro-ring resonators ("MRR”) 18, 19, 20, 21. The MZI system provides a readout parameter in phase or lambda (i.e. wavelength), a wavelength range of 1500 to 1600 nm, and a resolution of 1pm at the minimum.

The following sequence may be observed when a test is implemented with the MZI system. First, a drop of distilled water is added as a blank sample on the surface of the MZI system. Second, the droplet of distilled water is removed from the surface. Third, a drop of solution having a known concentration of Be ²⁺ is added on the surface. After 30 seconds to 2 minutes, the solution is removed from the surface. Then, a drop of distilled water is again added to the surface. Last but not least, laser is swept from a wavelength of 1500 to 1600 nm, and the output of the sensor is measured.

Referring to Fig. 7, the output of the sensor can be shown in a graph of transmission versus resonant wavelength. As the concentration of Be ²⁺ increases, the resonant wavelength tends to shift to the right, i.e. becomes higher. The shift may be up to 500 pm or more, and has been experimentally observed to be in a range of 340 to 430 pm, which can be used to identify presence of the Be ²⁺ and measure its concentration in a medium.

The above-mentioned method is relatively simple, but produces an optical sensor highly sensitive and selective in its ability to detect Be ²⁺ in an aqueous or non- aqueous environment from lOppb up to 200ppm. Compared to an ICP-MS or ICP- OES approach, a plurality of DAB9C3-coated resonant structures, e.g. mirroring resonators, disposed in a photonic platform, e.g. Mach-Zehnder interferometer, have a much smaller footprint. Despite the smaller footprint, the optical sensor is capable of detecting Be ²⁺ and measuring concentration of the same accurately. Moreover, the above-mentioned method employs non-toxic solvents. The optical sensor fabricated by the method, being compact, can be employed for on-site, in- situ detection and measurement of Be ²⁺ , hence is of a huge commercial value, particularly in the fields of rare earth mining, aerospace, nuclear and ceramic industries. It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications which does not affect the overall functioning thereof.